The long range goal of our research goal has been to develop and test rigorous theories of visual processing that address important aspects of behavioral and neural performance. [I] During the previous grant period we developed a very efficient method (the descriptive function method) for measuring the detection, discrimination and identification performance of single neurons. Briefly, [I] the responses the a neuron are measured along various stimulus dimensions, (2) descriptive functions are fitted to the means and standard deviations of the responses, and then (3) the fitted functions are used to determine single neuron performance. The high efficiency of this method allows us to measure the discrimination (or identification) performance of large populations of neurons and hence compare population performance to behavioral performance. The descriptive function method will be used to determine population performance for a number of stimulus dimensions. [II] Recent work in the primary visual cortex has revealed two important non-linear mechanisms, contrast normalization and response expansion, which make critical contributions to the discrimination and identification performance of cortical neurons. We propose a series of experiments and simulations to characterize the spatial, temporal, and noise properties of these mechanisms. These studies will be important for understanding how the normalization and expansion mechanisms contribute to psychophysical performance, and how they are implemented in the neural circuity of the retina, LGN, and cortex. [III] Quantitative models of using spatiotemporal sinewave granting stimuli. Typically, the stimuli are presented for a fixed duration, in counterbalanced blocks, with careful control of fixation. To test and develop general theories of spatial vision it is important to begin bridging the gap between these carefully controlled stimulus presentation conditions and the more complex stimulus presentations which occur in the natural environment. We propose a series of experiments which will measure neural responses and behavioral discrimination performance for sinewave grating stimuli presented in a fashion which matches the sequence of fixations during saccadic inspection of complex natural images. These data will be compared with results from more conventional presentation methods and will be used to develop quantitative models of spatial visions that are appropriate for natural visual tasks.